Section 6: Ion ImplantationIon Implantation - OverviewEquipmentIon ImplantationAdvantages of Ion ImplantationIon Implantation Energy Loss MechanismsIon Energy Loss CharacteristicsStopping MechanismsElectronic / Nuclear Stopping: DamageSimulation of 50keV Boron implanted into SiModel for blanket implantationProjected Range and StraggleSelective ImplantationTransverse (or Lateral) Straggle (Rt or R)Feature Enlargement due to lateral straggleDefinitions of Profile ParametersSelective Implantation – Mask thicknessTransmission Factor of Implantation MaskTransmitted FractionJunction DepthSheet Resistance RS of Implanted LayersApproximate Value for RSExample CalculationsChannelingUse of tilt to reduce channelingPrevention of Channeling by Pre-amorphizationKinetic Energy of Multiply Charged IonsMolecular Ion ImplantationImplantation DamageAmount and type of Crystalline DamagePost-Implantation Annealing SummaryDeviation from Gaussian TheoryShallow ImplantationRapid Thermal AnnealingDose-Energy Application SpaceEE143 – Ali JaveySection 6: Ion ImplantationJaeger Chapter 5EE143 – Ali JaveyIon Implantation - Overview•Wafer is Target in High Energy Accelerator•Impurities “Shot” into Wafer•Preferred Method of Adding Impurities to Wafers–Wide Range of Impurity Species (Almost Anything)–Tight Dose Control (A few % vs. 20-30% for high temperature pre-deposition processes)–Low Temperature Process•Expensive Systems•Vacuum SystemEE143 – Ali JaveyEquipment dttImqAQqrmVT021 Dose Implanted2 Field Magnetic x q particle chargedon ForceBBvFEE143 – Ali JaveyIon ImplantationxBlocking maskSi+C(x)as-implant depth profileDepth xEqual-ConcentrationcontoursReminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.yEE143 – Ali JaveyAdvantages of Ion Implantation•Precise control of dose and depth profile•Low-temp. process (can use photoresist as mask)•Wide selection of masking materials e.g. photoresist, oxide, poly-Si, metal•Less sensitive to surface cleaning procedures•Excellent lateral uniformity (< 1% variation across 12” wafer)n+n+Application example: self-aligned MOSFET source/drain regionsSiO2p-SiAs+As+As+Poly Si GateEE143 – Ali JaveyIon Implantation Energy Loss MechanismsSi++SiSiee++ElectronicstoppingNuclearstoppingCrystalline Si substrate damaged by collisionElectronic excitation creates heatEE143 – Ali JaveyLight ions/at higher energy more electronic stoppingHeavier ions/at lower energy more nuclear stoppingEXAMPLES Implanting into Si:Ion Energy Loss CharacteristicsH+B+As+Electronic stoppingdominatesElectronic stoppingdominatesNuclear stoppingdominatesEE143 – Ali JaveyStopping MechanismsEE143 – Ali JaveySubstrateLess damageSe > SnMore damage at end of range Sn > SeSurfacex ~ RpA+Eo = incidentkineticenergySeE ~ 0SnE=EoSeSnDepth xSn dE/dx|nSe dE/dx|eElectronic / Nuclear Stopping: DamageEE143 – Ali JaveySimulation of 50keV Boron implanted into SiEE143 – Ali JaveyModel for blanket implantation ppppppRNdxxNQRRRxNxN0p222= DoseStraggleRRange Projected2exp ProfileGaussian EE143 – Ali JaveyRp and Rp values are given in tables or charts e.g. see pp. 113 of JaegerNote: this means 0.02 m.Projected Range and StraggleEE143 – Ali JaveySelective Implantation solution ldimensiona-one is xNstraggle transverse2221,RRayerfcRayerfcyFyFxNyxNEE143 – Ali JaveyTransverse (or Lateral) Straggle (Rt or R) RtRtRp>1RtRpEE143 – Ali JaveyyMaskC(y) at x=Rpx = RpImplanted specieshas lateral distribution,larger than mask opening Implanted specieshas lateral distribution,larger than mask opening xyHigher concentrationLowerconcentrationFeature Enlargement due to lateral straggleEE143 – Ali Javey(2) Projected Range:(3) Longitudinal Straggle:(1) Dose C x dx0(4) Skewness: (5) Kurtosis: dxxCxRp01 dxxCRxRpp0221( ) ( )00,303<>-orMdxxCRpxM 04dxxCRxpRpxC(x)Kurtosis characterizes thecontributions of the “tail” regions -describes asymmetry between left side and right side Definitions of Profile ParametersEE143 – Ali JaveySelective Implantation – Mask thickness•Desire Implanted Impurity Level to be Much Less Than Wafer DopingN(X0) << NBorN(X0) < NB/10EE143 – Ali JaveyWhat fraction of dose gets into Si substrate?x=0 x=dC(x)Mask material (e.g. photoresist)Si substrateC(x)Mask material with d=x=0 x=d-Transmission Factor of Implantation MaskEE143 – Ali Javey T C x dx C x dxerfcd RRerfc x e dyC x dC x Rdppyxp 000412 212102Rule of thumb Good masking thickness:d R Rp p 4 3. ~are values of for ions intothe masking materialRpRp,Transmitted FractionEE143 – Ali JaveyJunction Depth BpppjBppjpBjNNRRxNRRxNNxNln22exp22The junction depth is calculated from the point at which the implant profile concentration = bulk concentration:EE143 – Ali JaveySheet Resistance RS of Implanted LayersxC(x) log scalexjCBnTotal doping concpp-sub (CB)n jx0BSdxCxCxq1RExample:n-type dopants implantedinto p-type substratex =0x =xjxEE143 – Ali JaveyApproximate Value for RS Rq C x dxqRqRohmsxssj1 110 If C(x) >>CB for most depth x of interest and use approximation: (x) ~ constantuse the for the highestdoping region which carriesmost of the currentThis expression assumes ALLimplanted dopants are 100%electrically activatedor ohm/squareEE143 – Ali Javey200 keV Phosphorus is implanted into a p-Si ( CB= 1016/cm3) with a dose of 1013/cm2 . From graphs or tables , Rp =0.254 m , Rp=0.0775m (a) Find peak concentrationCp = (0.4 x 1013)/(0.0775 x10-4) = 5.2 x1017/cm3(b) Find junction depths(c) Find
View Full Document
Unlocking...